The present invention relates in general to output drivers within integrated circuits (ICs), and in particular to current limited output driver circuits.
Many electronic devices require protection from the environs in which they operate. When certain physical conditions arise, such as the occurrence of opens in signal conductors or shorts to various power supply buses, the above conditions may impair device operation or even damage the device itself. For example, a power supply circuit designed to provide a specific operational current can be quickly damaged or destroyed if an excessive amount of load current is drawn. Similarly, communication and networking system circuits, such as line drivers and receivers, often encounter similar over-current conditions that may prevent the device from meeting certain interface specifications.
Depending on an electronic device's performance requirements, it is desirable to internally limit current either driven or drawn by the device. Internal current limiting circuitry eliminates the need for providing additional circuitry for external current limiting protection. To this end, output driver circuits have been designed to minimize adverse effects from excessive current when, for example, the output is short-circuited.
In certain applications, such as in networking systems, transceiver devices are used to drive and receive data along transmission lines in accordance with specific networking protocol such as V.28 (RS-232), V.35, RS449, EIA-530-A, X.21, etc. When transmissions lines are driven under load conditions, internally generated voltage levels correspondingly decrease as current is drawn by the load. If over-current conditions arise along the transmission lines, internal voltage levels can be corrupted, resulting in device malfunction. Furthermore, when subject to excessive current conditions, semiconductor structures within a device can be irreparably damaged. Hence, limiting output driver current below a target level is a significant goal in the design of output driver circuits.
A conventional approach senses the current of an output driver transistor directly to determine whether an over-current condition exists. That is, the output current itself is tapped into and is used to monitor over-current events. In a particular output driver circuit, a sensing resistor is placed in series with an output drive transistor to provide for "direct sensing" of the output current. Additionally, an over-current detecting circuit is coupled to the output driver circuit for measuring the voltage drop across the sensing resistor. In operation, when a certain amount of current flows through the sensing resistor, a potential difference develops across the resistor. The monitoring circuitry first compares the potential difference to a reference voltage and then determines whether an overcurrent condition exists. Upon detection of an over-current condition, the monitoring circuitry disables or "shuts off" the respective output drive transistor. The output current is thus limited by way of direct sensing.
FIG. 1 illustrates a common approach to current limiting output driver circuit 100 by way of directly sensing the output current. Output driver circuit 100 comprises pull-up driver circuit 112, pull-down driver circuit 111, input terminal 102 and output terminal 104. Since both driver circuits are structurally and functionally similar, the following discussion regarding the pull-down driver circuit applies to the pull-up driver as well.
Pull-down driver circuit 111 consists essentially of output drive transistor 106 (e.g., M1), sensing resistor 108 (e.g., R1) and over-current detecting circuit 110. In operation, a signal to be driven is received at input terminal 102 and driven out from output terminal 104. For example, when a relatively high voltage signal, such as +3.3 or +5 volts, is applied to input terminal 102, output drive transistor 106 activates to drive a relatively low voltage signal at output terminal 104. Correspondingly, M1 "sinks" output current ("Iout") 122 into output terminal 104 and through both M1 and R1 to change the output voltage level. If Iout is excessive, then the voltage (.DELTA.V) developing across R1 triggers the over-current detecting circuit to shut down the output driver circuit, thus alleviating the over-current condition.
A significant drawback to this approach is that the sensing resistor, in series with its respective output drive transistor, adversely affects the output signal integrity. To minimize such effects, the voltage drop across the resistor must be limited to a small amount, for example, 100 millivolts. To achieve this minimal voltage drop, the sensing resistor must have a relatively low resistance, such as approximately 1 ohm, or less. Manufacturing such resistors with acceptable accuracy and reliability, however, in present semiconductor processing technologies, such as CMOS, is both difficult and costly. Without reliably precise resistor values, there is a general risk of increased power dissipation associated with the voltage drop across the sensing resistor as well as a disruption in circuit operation due to degraded signal integrity. Signal degradation occurs, for example, when the magnitude of the output voltage swing is diminished due to the voltage drops across R1 and R2. Moreover, the decreased output voltage swings by a driver circuit leads to disruptions in device operation.
Furthermore, in applications requiring relatively high output currents, geometric sizes of both M1 and M2 must be large enough to reduce their output resistances. An increased size in output drive transistors, however, both increases the size of electronic device and limits the functionality of the electronic device.
Therefore, there is a need for a circuit and a method for indirectly sensing over-current conditions in output driver circuits that is not influenced by semiconductor process variations, that maintains output voltage signal integrity and does not consume more power than is necessary.